Oscillatory stratified flow over supercritical topography: Wave energetics and turbulence

Topographic features with steep, supercritical slope on the ocean bottom are sites of large energy conversion from the oscillating tide to internal waves according to linear theory. At the same time, they are also sites with potentially large local energy loss as is suggested by nonlinear waves and overturns found in field observations and two-dimensional simulations. Here, we investigate the internal wave dynamics and turbulence at an isolated supercritical obstacle using three-dimensional, high resolution large-eddy simulations (LES) conducted with a body conforming grid. An obstacle with a smoothed triangular shape having supercritical slope is considered as a laboratory-scale model for a two-dimensional ocean ridge. Three simulations are performed with constant Reynolds number and an excursion number (Ex) that varies from 0.066 to unity, corresponding to a large obstacle and a small obstacle, respectively. The dominant nonlinear flow feature is a downslope jet with intensified velocity whose length and thickness increase with increasing Ex. During the flow reversal phase, the downward jet encounters upward flow along the slope and the interaction creates a rebounding jet which distorts the density resulting in convective instability. Turbulence is generated predominantly by shear in the jet, convective instabilities from the rebounding jet, and breaking of transient lee waves. The active region of turbulence grows as Ex increases although the local normalized turbulent kinetic energy becomes smaller. Analysis of the baroclinic energy budget shows that, with increasing Ex, there is a substantial decrease of energy conversion (C) to the wave field as well as a substantial increase in local energy loss (q).